Enhancement: option to send HTTP over unix socket

add tests for unix socket connection

switch HTTP rest-server test to use any free port

allow rest-server test graceful shutdown opportunity

internal/backend/rest/rest_unix_test.go

@@ -0,0 +1,30 @@
+//go:build !windows && go1.20
+// +build !windows,go1.20
+
+package rest_test
+
+import (
+	"context"
+	"fmt"
+	"path"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestBackendRESTWithUnixSocket(t *testing.T) {
+	defer func() {
+		if t.Skipped() {
+			rtest.SkipDisallowed(t, "restic/backend/rest.TestBackendREST")
+		}
+	}()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	dir := rtest.TempDir(t)
+	serverURL, cleanup := runRESTServer(ctx, t, path.Join(dir, "data"), fmt.Sprintf("unix:%s", path.Join(dir, "sock")))
+	defer cleanup()
+
+	newTestSuite(serverURL, false).RunTests(t)
+}